1. Field of the Invention
The present invention relates to an electrostatic actuator, an electrostatic micro-relay and other devices using the same.
2. Description of the Background Art
FIG. 1 is an exploded perspective view that shows a structure of a conventional electrostatic actuator and FIG. 2 shows a cross-sectional view thereof. This electrostatic actuator 1, which is disclosed in Japanese Laid-Open Patent Application No. 2000-164104,is mainly constituted by a fixed substrate 2 and a movable substrate 3. The fixed substrate 2 is made of a glass substrate having a fixed electrode 4 and a pair of fixed contacts 5, 6 formed on the upper face thereof. The surface of the fixed electrode 4 is coated with an insulating film 7 made from an oxide. Moreover, the fixed contacts 5, 6 are respectively connected to connection pads 10, 11 on the fixed substrate 2 through respective wires 8, 9.
The movable substrate 3 made from an Si substrate is provided with a movable electrode 13 supported by four elastic beams 12 in the center portion thereof, and a movable contact 15 is placed on the center portion of the lower face of the movable electrode 13 through an insulating layer 14. An anchor 16 protrudes from a peripheral portion of the lower face of the movable substrate 3 so that, when the movable substrate 3 is fixed on the upper face of the fixed substrate 2 by the anchor 16, the movable electrode 13 and the fixed electrode 4 are aligned face to face with each other with a space in between; thus, the movable contact 15 is aligned face to face with the fixed contacts 5, 6 with a space in between in a manner so as to bridge a space between the fixed contacts 5 and 6.
In this arrangement, when a driving voltage, applied between the fixed electrode 4 and the movable electrode 13, has reached a predetermined voltage value, the movable electrode 13 is attracted toward the fixed electrode 4 side by an electrostatic attracting force that is exerted between the fixed electrode 4 and the movable electrode 13 so that the movable electrode 13 is allowed to adhere to the fixed electrode 4 through the insulating film 7 with the elastic beam 12 being distorted. In the case when the movable electrode 13 has adhered to the fixed electrode 4, before or after this process, the movable contact 15 is pressed between the fixed contacts 5 and 6 so that the fixed contacts 5, 6 are electrically closed by the movable contact 15 so that a pair of connection pads 10 and 11 are allowed to conduct to each other.
Therefore, in the case of an optimal electrostatic actuator, its CV characteristic is indicated by FIG. 3. Here, the CV characteristic of the electrostatic actuator is represented by the relationship between a driving voltage Vdrive applied between the fixed electrode 4 and the movable electrode 13 and a capacitance C between the two electrodes 4 and 13. In FIG. 3, C1 represents a value of the capacitance C in a state where no driving voltage is applied between the movable electrode 13 and the fixed electrode 4, C2 represents a value of the capacitance C in a state where the movable electrode 13 adheres to the fixed electrode 4 through the insulating film 7, on-voltage Von is a value of the driving voltage Vdrive at the time when the movable electrode 13 is made to adhere to the fixed electrode 4 (or is released from the fixed electrode 4), and in the case of an optimal electrostatic actuator, this CV characteristic has a symmetrical profile with respect to the point of the driving voltage Vdrive=0 volt.
In the case of a conventional electrostatic actuator, for example, the above-mentioned electrostatic actuator, when a driving voltage has been applied between the movable electrode and the fixed electrode for a long time, the insulating film on the fixed electrode is gradually charged, with the result that a variation occurs in operational voltage characteristics, such as on-voltage and off-voltage in the electrostatic actuator. Such a variation in the operational voltage characteristics is caused by the generation of an electrical potential difference other than the driving voltage Vdrive that is externally applied between the fixed electrode and the movable electrode for charging; therefore, when such a variation occurs in the operational voltage characteristics in the electrostatic actuator, the resulting problems are that the electrostatic actuator is not operated even when a rated on-voltage is applied thereto, and that the electrostatic actuator is not turned off even when the applied voltage is turned off. The following description will discuss the causes of variations in the operational voltage characteristics in detail.
The ways of charging are classified into two ways. Here, these ways are respectively referred to as a plus shift and a minus shift. The plus shift refers to a charging in which the center value of the CV characteristic is shifted toward the plus side of the driving voltage (see FIG. 6). The cause of the plus shift is that a charge transfer (transfer of charge) occurs at a portion on which the insulating film on the fixed electrode and the movable electrode come into contact with each other with the result that the insulating film is charged. The charge transfer is a phenomenon in which, when the contact portion of an insulator and a conductor is subjected to an electric field and heat, a charge is accumulated in the insulator, thereby charging the insulator.
For example, as shown in FIG. 4, in the case when a driving voltage Vdrive is applied between the movable electrode 13 and the fixed electrode 4, with the movable electrode 13 having a positive electric potential, at the contact portion between the movable electrode 13 and the insulating film 7, electrons e on the surface of the insulating film 7 are shifted toward the movable electrode 13 with holes h being left in the insulating film 7 so that the insulating film 7 is positively charged. However, in the case when the polarity of a driving voltage to be applied between the movable electrode and the fixed electrode is reversed so that the movable electrode has a negative electrical potential, the insulating film is negatively charged.
In the event of such a plus shift, the applied voltage Vapp between the movable electrode 13 and the fixed electrode 4 is lowered by a voltage ΔVp (>0) corresponding to the charge quantity due to the plus shift charging to a level represented by the following equation:Vapp=Vdrive−ΔVp,with the result that the apparent on-voltage is raised to Von+ΔVp (here, Von is a value of the on-voltage in the case of no charge). Therefore, the problem with the plus shift is exerted as an increase in the minimum driving voltage (apparent on-voltage) to be used for closing the fixed contacts 5, 6 by using the movable contact 15, and in the case of a great plus shift, the electrostatic actuator fails to turn on even when the rated voltage is applied.
Moreover, the minus shift refers to a charging in which the center value of the CV characteristic is shifted toward the minus side of the driving voltage (see FIG. 6). The minus shift is caused by an ionic charging. In other words, this is caused by the fact that ions, generated in processes such as an anodic bonding, are diffused in the insulating film of the oxide so that positive and negative ions diffused in the insulating film are shifted by an electric field applied between the movable electrode and the fixed electrode toward the mutually opposite sides.
For example, as shown in FIG. 5, in the case when a driving voltage Vdrive is applied between the movable electrode 13 and the fixed electrode 4, with the movable electrode 13 having a positive electric potential, anions p, diffused in the insulating film 7 of the oxide, are shifted in the direction toward the interface to the fixed electrode 4 with anions n being shifted in the direction of the surface of the insulating film 7 so that the surface of the insulating film 7 is negatively charged. However, in the case when the polarity of a driving voltage to be applied between the movable electrode and the fixed electrode is reversed so that the movable electrode has a negative electrical potential, the insulating film is positively charged.
In the event of such a minus shift, the applied voltage Vapp between the movable electrode 13 and the fixed electrode 4 is raised by a voltage ΔVn (>0) corresponding to the charge quantity due to the ionic charging to a level represented by the following equation:Vapp=Vdrive+ΔVn,with the result that the apparent on-voltage is lowered to Von−ΔVn (here, Von is a value of the on-voltage in the case of no charge). Therefore, the problem with the minus shift is exerted as a decrease in the minimum driving voltage (apparent on-voltage) to be used for opening the fixed contacts, with the result that, even when the driving voltage Vdrive is set to 0 volt, the electrostatic actuator fails to turn off or hardly turns off (that is, the electrostatic actuator is stuck, or susceptible to sticking).
In this manner, the insulating film is always charged while the electrostatic actuator is being driven, resulting in a failure to ensure the designed performances. FIG. 6 shows a change in the CV characteristic before and after a thermal endurance test carried out on an electrostatic actuator. The CV characteristic F0, indicated by broken lines and rhomboidal shape points in FIG. 6, represents an initial characteristic prior to the conduction of the thermal endurance test, which shows a characteristic that is symmetrical with the driving voltage Vdrive. The CV characteristics F+, F−, indicated by solid lines shown in FIG. 6, represent the CV characteristics after a thermal endurance test carried out under conditions of an ambient temperature of 85° C., a driving voltage of 24 volts and test time of 100 hours; and F+ indicated by solid lines and square points, represent plus shifts, while F−, indicated by solid lines and triangular points, represent minus shifts.